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Development, Commissioning, and Evaluation of a New Intensity Modulated Microbeam Proton Therapy System

J Farr

J Farr*, V Moskvin , R Lukose , S Tuomanen , M Gargone , P Tsiamas , F Pirlepesov , W Yao , St. Jude Children's Research Hospital, Memphis, TN


SU-I-GPD-T-181 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To invent, design, fabricate, and commission an intensity modulated microbeam proton therapy (IMµPT) system without the need for physical collimation and compare its resulting conformity to a conventional IMPT system.

Methods: A proton therapy system (Hitachi, Ltd, Hitachi City, Japan; Model: Probeat-V) was specially modified to produce scanned microbeams without collimation. Integral depth dose acquisitions and calibrations were performed with a large diameter parallel plate ionization chamber in a scanning water phantom (PTW, Freiburg, Germany; Models: Bragg Peak ionization chamber, MP3-P). Spot size and shape was measured using radiochromic film (Ashland Advanced Materials, Bridgewater NJ; Type: EBT3), and a synthetic diamond diode type scanned point by point in air (PTW Models: MicroDiamond, MP3-P). The measured data were used as inputs to generate a Monte Carlo based model for a commercial radiotherapy planning system (RTPS) (Varian Medical Systems, Inc., Palo Alto, CA; Model: Eclipse v13.7.15). The regular ProBeat-V system (sigma ~2.5 mm) RTPS model was available for comparison. A simulated base of skull case with small target proximal to brainstem was planned for both systems and compared.

Results: The spot sigma is determined to be 1.4 mm at 221 MeV. Integral depth doses were indistinguishable from the standard spot commissioning data. The RTPS fit the spot profiles closely, giving a residual error maximum of 2.5% in the spot penumbra tails (below 5% of maximum) from the commissioned energies 69.4 to 221.3 MeV. The resulting IMµPT plan is significantly more conformal than the IMPT plan due to a sharper dose gradient (90-10%) 1.15 mm cf. 0.75 mm, IMPT and IMµPT, respectively, at a central water equivalent depth of 7 cm.

Conclusion: A new IMµPT system was developed, implemented and tested. Initial results look promising in cases where small target volume treatments can benefit from additional dose sparing to neighboring sensitive structures.

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